Riveted structures such as the wing panels and body panels of an airplane are frequently assembled with structural members tack riveted to skins for subsequent production riveting on an automatic riveting machine. In keeping with high strength-to-weight construction, the skins and structural members, viz, stringers, ribs, frames, etc. are usually of an aluminum alloy material secured together with aluminum alloy rivets. The rivets used for this purpose are generally of two alloy types designated and known commercially as 2117S or 2024S. Of these, 2024S rivets are preferred for their superior physical properties, which after heat treatment and age hardening, develop physical properties somewhat in excess of the properties of low carbon steel. The heat treatment for 2024S material is accomplished in the conventional manner as for other metals, i.e., the material is heated to a certain temperature and thereafter rapidly cooled by quenching in water. The 2024S material is soft after quenching but attains 90% of its maximum hardness and strength after aging for 24 hours at room temperature, and full strength after 4 to 6 days. This age hardening is very rapid in the first 4 hours after quenching, necessitating storage at very low temperatures to retard or delay the age hardening process until such time as the rivets are to be used. After approximately 20 minutes exposure at room temperature, the 2024S rivets attain a degree of hardness sufficient to prohibit their being used without danger of shearing or developing cracks.
In the handling of 2024S rivets, refrigerated containers are usually placed in convenient location within close proximity to the work area where they are needed. Rivets are quick frozen immediately following their fabrication and transported in insulated containers to the refrigerators for storage until such time as they are to be used. Workmen subsequently withdraw the frozen rivets in limited quentities to conserve supplied and avoid undue scrappage as a result of premature age hardening at room temperature.
Aircraft structures such as wing and fuselage panels, are frequently assembled with 2024S rivets in order to accommodate the load requirements of a particular design. The panels are usually hand riveted by the well-known rivet gun/bucking bar method, or wherever the design permits, by hand operated rivet squeezers. Hand riveting is obviously expensive and slow as compared to riveting done on automatic riveting machinery. However, presently available automatic riveting machines are not equipped to handle the 2024S type or "ice box" rivets.
The automatic riveting machines of the type employed by the aerospace industry are equipped with a plurality of motor driven hoppers in which a supply of rivets is contained subject to being dispensed by associated dispensing mechanism in individual units. Each hopper may contain a particular diameter and length rivet as required to secure the individual components of a workpiece together. The present invention adapts these automatic riveting machines for use with ice box or 2024S rivets. As presently constructed, i.e., without benefit of refrigeration or moisture inhibitors, rivets contained in the rivet hoppers would instantly freeze into clusters or agglomerates upon exposure to atmospheric humidity to thereby render the hopper's dispensing mechanism completely inoperative.